Traditional offset printing does not allow for variable data printing. The inking subsystem used applies ink over a static plate image. Typically, ink is depleted from an inker form roll as the ink is transferred onto the imaging plate, the ink form roller being the last roller that is in direct contact with the imaging plate. Different regions of the imaging plate may need more or less ink depending upon which regions are oleophilic foreground areas and which regions are oleophobic background image areas.
Traditional offset ink delivery systems adjust ink flow to different regions of the plate using manually adjusted keys which change the ink feed rate in order to guarantee enough ink will flow in solid imaging regions but prevent too much ink from flowing to areas covered by fine lines or half tones.
Recently, keyless inker systems have been introduced which meter ink appropriately without the need for inker keys. Exemplary keyless inker systems include those sold by Koenig & Bauer AB group (KBA) located in Germany. Such keyless systems use a metered anilox roller to pull fresh ink uniformly out of an ink tray and deliver the ink directly to a rubber form roll which then transfers the ink to the an imaging plate. Such systems provide for more consistent ink flow regardless of whether a solid or fine artwork is being printed. However, the layer of ink remaining on the form roller after being partially transferred to the static image on an offset plate has a thickness that is not uniform. This is because ink splits onto the imaging plate in imaging areas but is fully rejected in non-imaging areas by the dampening fluid. Thus, the remaining non-uniform ink thickness on the form roller has a thickness pattern which reflects the image pattern printed onto the static plate. Thus, not all areas on the form roll are covered with the same thickness or amount of ink after transfer of ink onto the imaging plate and when new ink is transferred onto the form roller some of the old ink pattern partially remains.
To minimize these effects, keyless inking systems include a form roll that has a soft or conformable surface, an anilox metering roll, and an imaging plate that are all substantially equal in diameter. Further, since these rollers are all of equal diameter, related art keyless inking systems typically have large diameter anilox meter rollers and form rollers since the image plate is large in area, for example a B2-size sheet format. These rollers traditionally are of equal diameters so that ink imaging history effects are added “in-phase” with the image on the plate. The form roller then builds up a reproducible ink layer thickness “in phase” with the static offset plate image, which unfortunately leaves ghosting between print jobs.
For a variable data lithographic printing inker system, the ink film thickness must always be the same regardless of the imaging history because a new image may be introduced on each pass of the printing process. New images are introduced based on a new pattern of dampening solution formed by laser evaporation on each pass of the imaging cylinder containing a reimageable print surface. In addition, variable data lithography is different from static offset lithography because the ink is transferred directly to an elastomeric conformable blanket that holds the latent image in the dampening fluid after it has been laser patterned in contrast to traditional offset which holds a static fluid pattern over a hard metal offset plate surface. Thus, a new inker system is needed that is compatible with the unique requirements of a variable data lithography print system.
Efforts have been made to create lithographic and offset printing systems for variable data. One example is disclosed in U.S. Pat. No. 9,216,568 published Dec. 22, 2015, which is commonly assigned, and the disclosure of which is hereby incorporated by reference herein in its entirety, in which a chamber blade system is configured to supply ink to an anilox member of an inking system. The inking system includes a soft ink transfer roll and a hard form roll. Ink is transferred from the anilox roll to the form roll by way of the transfer roll, and from the form roll to a reimageable surface layer of an imaging member of a variable data lithographic system. An ink layer free of ink history is uniformly applied onto a surface of the form roll, and subsequently transferred to the reimageable surface layer while avoiding or substantially eliminating image ghosting. However, the inventors have discovered that it would be beneficial to provide further improved printing systems and methods for printing higher viscosity digital variable lithography highly pigmented inks.
Flexography (flexo) printing known to the inventors use anilox rollers in meter flexo inks in the 10 centipoise (cps) to 1,000 cps range. Anilox roll patterns include hexagonally packed ink well cells or trihelical grooves at a 30, or 60 degree angles. Such patterns come in various shapes and sizes and line screens (e.g., the minimum repeat distance). However, the high rheology inks utilized in digital variable lithography are not characteristic of traditional flexo inks. Hot temperature (e.g., at least about 60 degrees C.) rheology is in the range of 100,000-1,000,000 cps, and viscosity is above 1 million cps at lower temperatures (<40 degrees C.). The inventors found that none of these standard anilox engraving patterns adequately meter these inks due to high hydrodynamic pressures arising from their very high viscosity (e.g., at least 100,000-1 million cps).